Energy storage systems using large superconducting magnets have been proposed for leveling daily load requirements on electrical utility systems. Excess energy generated during off-peak hours can be stored and later returned to the power grid during high demand periods. By connecting the superconducting energy storage magnet to the power system with a bridge-type inverter, it is possible to obtain very efficient energy transfer between the storage magnet and the power system, as more fully described in U.S. Pat. No. 4,122,512 to Peterson, et al., incorporated herein by reference.
The large energy storage magnets proposed for storing sufficient energy to allow load leveling on a power grid utilize multiple turns of composite conductor formed of normal and superconducting material. The current flowing in the turns of the magnet naturally produces a net magnetic field and any conductor in the field will experience a force at each point on the conductor oriented at right angles to the current and the magnetic field. Since superconducting magnets of the size proposed for electrical system energy storage will conduct extremely large currents and will generate strong magnetic fields, the forces experienced by the conductors will be very large. Since no conductor by itself could possibly withstand the forces that would be exerted on the conductor under these conditions, an external support structure capable of resisting the large loads imposed on the conductor is thus necessary.
However, substantial practical difficulties are encountered in supporting the superconducting magnet because of the supercooled conditions under which the magnets must be operated. For example, the support structure must not add a significant thermal load on the cooling system.
Further, the system must be capable of adjusting to the expansions and contractions encountered during the initial cool-down of the system and any subsequent heating and cooling cycles. For example, the system would preferably be built and installed in a warm structure, such as bedrock, which is at normal ambient temperatures (e.g., 50.degree. to 70.degree. F.). The system must therefore be designed to withstand the thermal stress caused by the temperature change from ambient installation temperatures to cryogenic operating temperatures (approximately 4K). The system must be able to accommodate the thermal stresses due to the cool-down process to ensure structural reliability.
One approach to the problem of adequately supporting a superconducting energy storage magnet is shown in U.S. Pat. No. 3,980,981 to Boom, et al., incorporated herein by reference. The structure disclosed in that patent includes a composite superconducting-normal conductor which is rippled radially with the ripples lying in a plane normal to the net magnetic field experienced by the conductor. The ripples are designed to absorb the magnetically induced stresses associated with charge up of the magnet. Support columns extend radially to an outer support wall which may be formed in bedrock. The columns are made of insulating material and the necessary thermal shielding Dewar is accommodated around the conductor with minimal interference from the radial support members.
Another approach to the problem of adequately supporting a superconducting magnet is shown in U.S. Pat. No. 4,622,531, issued to Eyssa, et al., incorporated herein by reference. The structure disclosed in this patent includes two separate coils of one layer each of composite superconductor, disposed so that the forces experienced by the conductors at each point on the conductor are directed primarily inward toward the other conductor. The conductors are also rippled in a direction normal to the net magnetic field to absorb the magnetic stresses.
A further approach is described in related U.S. application Ser. No. 07/586,496 by Boom, et al. filed on Sep. 21, 1990, now U.S. Pat. No. 5,237,298, issued Aug. 17, 1993. A system is disclosed therein having vertically stacked coils disposed at an angle with respect to the axis of the coil in a trench dug into bedrock. The coils are also rippled with the undulations lying in planes which are substantially orthogonal to the axis.
Previous SMES systems generally have been designed to accommodate thermal and magnetic stresses within the support structure. Compromises between acceptable thermal and magnetic stresses were required to provide a system with the smallest acceptable bending stiffness. This compromise generally can result in significant working (straining, lengthening, bending) of the conductors in such systems, and this working of the conductors due to magnetically induced forces increases electrical resistance over time, and thereby decreases stability. Prior support designs typically require many struts to transfer the net radial forces to the warm structure (bedrock) and the coils are relatively high, requiring deep trenches. Further, special mechanisms are often necessary to accommodate the large vertical displacement of the magnet structure due to structure cooldown and axial magnetic loading.